1. Field of the Invention
The invention relates to a communication of information between cell-site stations and mobile units in a Code Division Multiple Access (CDMA) cellular system, more particularly to a method and apparatus for providing a CDMA-to-CDMA different frequency hand-off in a CDMA cellular telephone system when a mobile station travelling in several service regions requires a hand-off from a source cell-site to a target cell-site which is using a frequency band different from that of the source cell-site.
2. Description of the Prior Art
In the conventional cellular mobile telephone system, the cellular service area is divided into several sub-areas, each sub-area being provided with base stations (also known as cell-site stations or cell-sites or for short cells) for controlling cellular services so that a large number of mobile units, e.g. car telephone or cellular telephone, may be serviced with radio communication. In this case, the cellular telephone system requires a number of channels in order to provide radio communication services for a large number of mobile telephone system users, and Code Division Multiple Access and Time Division Multiple Access are typical of such channel securing techniques.
However, as the case may be, in order to accommodate multichannel or to improve the capacity of overall system it is necessary to design a cellular telephone system using the Frequency Division Multiple Access technique. The use of FDMA results in a base station using a particular frequency band which is not assigned to other surrounding base stations.
A hand-off of a mobile telephone in a CDMA system is disclosed in U.S. Pat. No. 5,101,501, entitled "METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATION IN A CDMA CELLULAR TELEPHONE SYSTEM" and "CDMA digital CAI Standard--CELLULAR SYSTEM DUAL-MODE MOBILE STATION--BASE STATION COMPATIBILITY STANDARD, DRAFT REVISION 1.0" published on Oct. 1, 1990.
In the above mentioned US patent, a cellular telephone system using a soft hand-off scheme (a make-before-break switching (function) is proposed which is suitable for use in such a case that two neighboring cell-sites use the same frequency band. The soft hand-off from one cell to another is initiated when the mobile receiver recognizes that the received pilot signal strength from the cell-site receiver handling a call falls below a predetermined threshold value. During the hand-off, a mobile unit receives user information emanating from two base stations, and achieves better receiving sensitivity by multi path diversity combining and after then switches the channel from one base station to the other station to complete the hand-off operation. But a CDMA cellular telephone system which includes a FDMA scheme can not carry out a hand-off operation of the mobile unit using such a soft hand-off, because if a base station to which a mobile unit is going to switch or hand-off does not use the same frequency that the mobile unit currently uses, this mobile unit can not receive any information from the base station to be handed-off.
In the above cited CAI standard, two approaches for solving possible problems of a different frequency hand-off scheme in the CDMA cellular telephone system are recommended. The one approach adopts a method in which a home cell-site presently servicing the call instructs the mobile unit to hand-off in response to the frequency occupation state used by neighboring cell-sites and the distance between the home cell-site and the mobile telephone unit. But this method has some disadvantages in that in a certain direction along which the mobile unit moves, an undesirable hand-off may occur because the influence of the intensity of the propagation signal is not taken into consideration as the distance between the cell-site and mobile unit is the only decision parameter for determining if the hand-off is initiated or not. Moreover, the hand-off operation may not be accomplished even in a situation when hand-off is required to be initiated.
On the other hand, in the second approach recommended in the above CAI standard, neighboring cell-sites continuously transmit pilot signals through a frequency channel used by the cell-site presently providing service, but this approach does not communicate user information through the frequency channel. As a result, a hand-off is determined by the same method as the soft hand-off proposed in the above U.S. Pat. No. 5,101,501, and a different frequency hand-off instruction can be issued to the mobile unit in response to the hand-off determination and the frequency usage state of neighboring cell-sites. But unfortunately, additional communication equipment is necessary to realize this method which is too expensive, because most of all the equipment contained in the system needs to be used even in transmitting the pilot signal.
As apparent from the above discussion, the conventional prior art methods are not appropriate in design for use with a CDMA cellular telephone system which includes FDMA for improving the overall capacity of the system.